As clock and logic speeds increase, the power requirements of CMOS circuits are rapidly becoming a major concern in the design of personal information systems and large computers. In portable systems, the computational time between battery recharge is limited by a preference for light weight batteries. With large multiprocessor systems, dissipation of the heat from a large volume of processors is a problem. Further, the energy costs become more significant.
In principle, a computing engine need not dissipate any energy, as shown in the work of Landauer, Bennet, and Feynman (Landauer, R., "Uncertainty Principle and Minimal Energy Dissipation in a Computer", International Journal of Theoretical Physics, Vol. 21, Nos. 3/4, 1982, pages 283-297; Bennet, C., "The Thermodynamics of Computation--a Review", International Journal of Theoretical Physics, Vol. 21, No. 12, 1982, pages 905-940; and Feynman, R., "Quantum Mechanical Computers", Foundations of Physics, Vol. 16, No. 6, 1986.) Although these authors approach the problem from different disciplines and use different physical as well as theoretical models, they all conclude that the transfer of energy through a dissipative medium such as a switch dissipates arbitrarily small amounts of energy if this transfer is made sufficiently slowly. This should not come as a surprise: to eliminate the dissipation we have to make the potential applied to the medium zero, and from thermodynamics we know that the only way to transfer energy with zero potential is to allow the transfer to happen over an infinite amount of time.
Power dissipation in conventional CMOS primarily occurs during device switching. One component of this dissipation is due to charging and discharging the gate capacitance through a conducting, but slightly resistive, device. We note here that it is not the charging or the discharging of the gate that is necessarily dissipative, but rather that a small time is allocated to perform these operations. In conventional CMOS, the time constant associated with charging the gate through a similar transistor is RC, where R is the ON resistance of the device and C its input capacitance. However, the cycle time can be, and usually is, orders of magnitude longer than RC. A conclusion is that power consumption can be reduced by spreading the transitions over the whole cycle rather than "squeezing" it all inside one RC. It is this observation that is the core of a number of proposals to construct low power electronic computing engines. By low power, or non-dissipative, we mean that the energy per computational step can be made arbitrarily small by spreading the computation over a longer period.
Fredkin and Toffoli (Fredkin, E. and Toffoli, T., 1978, "Design Principles for Achieving High-performance Submicron Digital Technologies," Proposal to DARPA, MIT Laboratory for Computer Science) demonstrated one realization of a low power universal gate using conservative logic. In conservative logic, the information content as well as the number of 1's and 0's are conserved throughout the computation. One property of a computation using conservative logic is the production of unwanted intermediate outputs. Unfortunately, discarding these outputs results in energy dissipation. Recycling it however, does not. It is this operation of recycling that requires the use of reversible logic (Fredkin, E., and Toffoli, T., "Conservative Logic, "International Journal of Theoretical Physics, Vol. 21, Nos. 3/4, pages 219-253). The CMOS gate proposed by Fredkin and Toffoli could not easily be integrated, however, as it requires the use of inductors internal to the computational network. The sizes and numbers of these inductors are well beyond what can be easily accommodated on a silicon substrate.
Aware of the requirement to spread the energy transfer over a longer period of time, Seitz et al. (Seitz, Charles L. et al , "Hot-Clock nMOS," in Proceedings of the 1985 Chapel Hill Conference on VLSI, Computer Science Press, 1985.) proposed a new reduced-power CMOS design style. The authors elected to use only N-Channel devices and therefore depended on bootstrapping action in order to eliminate the V.sub.T voltage drop through their devices. The authors having successfully fabricated and operated numerous circuits using this style still warned of the importance of device sizing to achieve enough bootstrapping for proper operation.